The present invention relates to a process for partially oxidizing acrolein to acrylic acid in the gas phase under heterogeneous catalysis by conducting a starting reaction gas mixture which comprises acrolein, molecular oxygen and at least one inert gas containing at least 20% by volume of molecular nitrogen and contains the molecular oxygen and the acrolein in a molar O2:C3H4O ratio of ≧0.5 in one reaction stage over a fixed catalyst bed which is arranged in two spatially successive reaction zones A,B, the temperature of reaction zone A being a temperature in the range from 230 to 320° C. and the temperature of reaction zone B likewise being a temperature in the range from 230 to 320° C., whose active composition is at least one multimetal oxide comprising the elements Mo and V, in such a way that reaction zone A extends to an acrolein conversion of from 45 to 85 mol % and, on single pass of the starting reaction gas mixture through the overall fixed catalyst bed, the acrolein conversion is ≧90 mol % and the selectivity of acrylic acid formation, based on acrolein converted, is ≧90 mol %, the chronological sequence in which the starting reaction gas mixture flows through the reaction zones corresponding to the alphabetic sequence of the reaction zones.